1. Field of the Invention
This invention relates to an improved high-voltage device and, more particularly, to an improved high-voltage switch. More specifically, the present invention relates to an improved high-voltage interrupter switch and to switchgear utilizing the improved switch. The present invention represents an improvement over the switch constructions disclosed in the following commonly-assiged U.S. Pat. Nos. 4,169,973; 3,676,629; 3,671,697; 3,567,967; and 3,549,840.
2. Discussion of the Prior Art
The above commonly-assigned patents relate in varying degrees of particularity to a high-voltage interrupter switch. The switch includes a stationary contact assembly connectable to one side of a high-voltage source and a switch blade connectable to the other side of the source. The switch blade is rotatable into and out of engagement with a stationary contact portion of the stationary contact assembly. The stationary contact is enclosed within a closed chamber defined by an arc-extinguishing housing, sometimes referred to as an arc-compression chute. The housing is made of an ablative arc-extinguishing material, such as a polyurea formaldehyde sold under the trademark Delrin. In moving into and out of engagement with the stationary contact, the switch blade moves through the closed chamber defined by the housing and into and out of a slit formed in the housing communicating with the chamber. The stationary contact assembly and the housing are mechanically attached so that the stationary contact is within the chamber.
Assuming the switch blade to be engaging the stationary contact (switch closed) and the switch to be energized (connected to the voltage source and conducting load current), if the switch blade is rotated away from the stationary contact (switch open) an arc forms between the blade and the stationary contact. The arc impinges on the walls of the chamber defined by the arc-extinguishing housing and evolves therefrom arc-extinguishing gas confined by the closed chamber which de-ionizes, cools and extinguishes the arc. All of the above is well known and is set forth in the above commonly-assigned patents.
Large numbers of switches of the above-described type have been in use for a number of years. Recently, it has been discovered that some of these switches, typically those used in tropical or very humid environments, are experiencing deterioration caused by erosion and decomposition of a portion of the arc-extinguishing housing. The deterioration, if severe enough, may permit the chamber to be partially open to the environment external of the housing which is undesirable because the chamber is intended to be more or less closed and sealed. If the chamber is not closed or sealed, its ability to extinguish arcs formed between the blade and the stationary contact assembly can be compromised. When the above-described deterioration was first detected, the cause therefor was unknown. After prolonged study and testing, the following probable causes and expanations for the deterioration have been postulated.
The deterioration was found to be the heaviest on external surface regions of the housing which were close to or contiguous with those portions of the housing to which the stationary contact assembly was mechanically attached. Study, testing, and analysis indicated that the deterioration of these surface regions is effected by low magnitude leakage current flowing across the surface regions through their distributed surface resistances r.sub.n to ground through small distributed stray capacitances c.sub.g of the entire surface (see FIG. 6) directly from the stationary contact assembly. It was found that these leakage currents, at the voltages involved, could result in partial discharges or arcing on the surface regions of the housing. For purposes hereof, "partial discharge" means one or more electrical discharges that bridge a small part of the surface regions experiencing deterioration. The material of which the housing is made--dictated primarily by the need for the housing to have arc-extinguishing characteristics--was found to be eroded by the partial discharges. In tropical environments, the housing is also decomposed by ozone, oxides of nitrogen and acids. The ozone and nitrogen oxides are produced by the breakdown of air by the partial discharges. The acids are formed by a combination of the oxides of nitrogen and the water found in tropical environments. The water may be in either vapor form or in a condensed layer on the housing.
As the analysis and testing continueed, it was noted that the surface regions undergoing erosion and deterioration are located between distinct two portions of the housing. One of those portions has been previously described, and is that portion of the housing to whch the stationary contact assembly (connected to one side of the voltage source) is mechanically attached. The other portion of the housing is physically remote from the one portion, except for the small, distributed stray capacitance c.sub.g to ground; it is isolated both from ground potential and from the other side of the voltage source. Thus, the voltage of the source, which is connected to the switch blade and to the stationary contact assembly, is essentially not impressed across the two housing portions. Accordingly, this second portion of the housing, which generally constitutes a tip or free edge thereof, is essentially electrically floating. Because the second portion of the housing is essentially electrically floating and does not have the voltage of the source impressed across it and the first portion, the magnitude of leakage current flowing across the surface region through the distributed surface resistances r.sub.n to ground through the small stray capacitance c.sub.g is quite small but is, nevertheless, sufficiently large at high voltage to result in the aforementioned partial discharges which result in the concomitant deterioration of the housing.
All of the above has led to the conclusion that, if a surface region, which would otherwise experience erosion and deterioration, were to be supplied with current from other than the stationary contact assembly, and if the leakage current could be prevented from flowing across the surface region directly from the stationary contact assembly, the deterioration could be eliminated or reduced. It was postulated that capacitively supplying current to the surface region could prevent the partial discharges. Such capacitive coupling of current to the surface region, which current would not flow directly from the stationary contact assembly across the surface region, and would, therefore, produce little or no voltage drop across the surface region, it was further postulated, would prevent adjacent segments of the surface region from having sufficient potential to initiate partial discharges, that is, the surface region would experience a voltage gradient insufficient to initiate partial discharges.
These conclusions and postulations proved correct; closely capacitively coupling current to the surface region through large distributed capacitances c.sub.n (FIG. 6) with a conductive projection closely spaced (typically from about 1/16" to about 3/8" ) from the region and electrically connected to the stationary contact assembly, indeed, eliminates or reduces deterioration hereof. Placing the projection on the surface region merely moves the deterioration problem to those surface regions adjacent its terminus. Placing the projection too far from the surface region--say, 6 inches--produces too small a capacitance between the projection and the surface region to supply significant capacitive current to the surface region; the majority of the current supplied to the surface region is still supplied directly from the stationary contact assembly and the problem of deterioration remains.
Certain types of prior art shields are not suggestive of the above conclusions and postulations, nor of the desirability of using the conductive projection, because these prior art shields themselves, and the devices with which they are used, differ structurally and functionally from, and produce different results from, the present invention.
Specifically, it is known to include a corona or grading shield directly on or beneath the outer surface of the insulative housing of a high-voltage fuse. Each end of the fuse housing mounts a conductive end ferrule, with one of which the shield is electrically continuous. Unlike the switch housing discussed above, one end of the fuse housing is not at or near the potential of a high-voltage source with the other end of the fuse housing electrically floating: Before the fuse operates, both ends of its housing (and the end ferrules) are at the same high voltage as the source; after the fuse operates, the full high voltage of the source is impressed across the ends (and the ferrules) and, therefore, across the entire fuse housing. Moreover, because the full voltage of the source is impressed across the ends of the fuse housing, leakage currents thereacross can be much higher than those across the surface of the switch housing of the present invention. A fuse shield also is not used to closely capacitively couple current to the fuse housing; its function is to grade the electric field about the fuse and between internal fuse elements and the other end ferrule after operation thereof to prevent external flashover. Thus, the location of a fuse shield is not necessarily critically related to the location of the surface of the fuse housing. Its location is, rather, related to the size, configuration, and extent of those internal fuse elements which are electrically continuous with the one ferrule so that voltage stress exteriorly of the fuse is insufficient for a ionized path to form through the air surrounding the fuse housing.
It is also known to include a corona or grading shield on an insulator or insulator string. Similar to a fuse after its operation, the ends of the insulator or insulator string have high voltage impressed thereacross; typically one end is at the voltage of a line or cable supported thereby while the other end is at the ground potential of a tower or other structure connected thereto. Again, the character of the voltage impressed across an insulator or insulator string and of leakage current across the surface thereof vary from the voltage impressed across and the current flowing across the surface of the switch housing of the invention. The function of the insulator shield is to more evenly distribute voltage stress over the insulator or insulator string. To this end the shield, which has a flared, bell-shaped configuration, is mechanically connected to and surrounds the end of the insulator and is electrically connected to the line or cable. The shield is not, as is the projection of this invention, closely spaced from the insulator surface, but flares away therefrom.